1. Field of the Invention
The present invention relates to image forming methods, image forming apparatuses and an inkjet head and, more particularly, to an inkjet image forming apparatus, such as a printer, a copy machine, a facsimile machine, or a multi-function peripheral having the functions thereof, and an image forming method used in the image forming apparatus.
2. Description of the Background Art
As shown in, for example, FIGS. 2 and 3, an image forming apparatus, such as an on-demand inkjet printer, employs an inkjet head including a pressure chamber 2 filled with ink. The pressure chamber 2 is connected to a nozzle 3. When the pressure chamber 2 is filled with ink, an ink meniscus (hereinafter referred to simply as “meniscus”) is formed in the nozzle 3. The meniscus is a boundary between liquid contained in a thin pipe or the like and surrounding gas, and generally means a surface of the liquid. A piezoelectric element 9 which deforms when a drive voltage is applied thereto and a vibrating plate 7 which is laminated together with the piezoelectric element 9 are provided on the pressure chamber 2 on a side opposite the nozzle 3. The piezoelectric element 9 and the vibrating plate 7 form a driving unit D.
In the inkjet head, the driving unit D transmits a force generated as a result of deformation of the piezoelectric element 9 to the ink contained in the pressure chamber 2 as pressure. Thus, the driving unit D serves as a drive source for ejecting an ink droplet from the nozzle 3 connected to the pressure chamber 2. More specifically, the driving unit D deforms the piezoelectric element 9 by applying a drive voltage thereto, so that the vibrating plate 7 bends toward the pressure chamber 2, as shown by the dot-dash lines in FIG. 2. Accordingly, the capacity of the pressure chamber 2 is reduced and the ink contained in the pressure chamber 2 is pressurized. As a result, the ink is ejected from an end of the nozzle 3 as an ink droplet. The vibrating plate 7 also bends in a direction opposite to the direction shown by the dot-dash lines in FIG. 2 when the vibrating plate 7 receives pressure from the ink contained in the pressure chamber 2. Thus, the driving unit D also serves as an elastic element with respect to the vibration of the ink.
When a voltage is applied to the piezoelectric element 9 and stress is generated, the ink receives a pressure from the driving unit D through the vibrating plate 7 and starts to vibrate. In the vibration of the ink, the driving unit D and the pressure chamber 2 serve as elastic elements. A supply hole 5 through which the ink is supplied to the pressure chamber 2, an ink channel 4 which connects the pressure chamber 2 to the nozzle 3, and the nozzle 3 serve as inertial elements. The natural vibration period of the volume velocity of the ink in each of the above-mentioned sections is determined by the dimensions of each section, the physical properties of the ink, and the dimensions and physical properties of the driving unit D. In the piezoelectric inkjet head, the vibration of the ink is generated so that the meniscus in the nozzle 3 also vibrates, and thereby the ink droplet is ejected.
In the inkjet head having the above-described structure, a constant drive voltage is continuously applied to the piezoelectric element 9 in a non-printing state so that the piezoelectric element 9 is continuously deformed and the vibrating plate 7 is continuously bent. Thus, the state in which the capacity of the pressure chamber 2 is reduced is maintained. In a printing operation, the following driving method is generally used. First, the drive voltage is reduced to 0 so that the deformation of the piezoelectric element 9 and the bending of the vibrating plate 7 are canceled immediately before printing is started. Accordingly, the capacity of the pressure chamber 2 increases and the ink meniscus in the nozzle 3 is temporarily pulled toward the pressure chamber 2. Second, the drive voltage is applied to the piezoelectric element 9 again so that the piezoelectric element 9 is deformed and the vibrating plate 7 is bent toward the pressure chamber 2. Accordingly, the capacity of the pressure chamber 2 decreases and the ink droplet is ejected from the end of the nozzle 3. This driving method will sometimes be referred to as “the pull-push driving method” in the following description.
FIG. 12 is a schematic graph illustrating the relationship between the fluctuating voltage wave of the drive voltage Vp (shown by the dot-dash curve) applied to the piezoelectric element 9 in the above-described pull-push driving method and the variation in the volume velocity of the ink (shown by the solid curve L) in the nozzle 3 when the fluctuating voltage wave is applied. The fluctuating voltage is the voltage measured between an electric power source (not shown) and the piezoelectric element 9. With regard to the volume velocity of the ink, the positive sign shows the direction toward the end of the nozzle 3 and the negative sign shows the direction toward the pressure chamber 2. Here, a case is considered in which the thin, plate-shaped or layered piezoelectric element 9 shown in FIGS. 2 and 3 is used. When a fluctuating voltage is applied, the piezoelectric element 9 vibrates in a transverse vibration mode in which the piezoelectric element 9 expands and contracts in a planar direction.
The fluctuating method will be described with reference to FIG. 12. In a standby state before time 0 at the left end in FIG. 12, the drive voltage Vp is maintained at VH (Vp=VH) so that the piezoelectric element 9 is continuously contracted in the planar direction. Therefore, the vibrating plate 7 is continuously bent in a certain shape so that the state in which the capacity of the pressure chamber 2 is reduced is maintained. Before time 0, the ink is in the stationary state. In other words, the volume velocity of the ink in the nozzle 3 (line C) is maintained at 0 and the meniscus in the nozzle 3 is stationary.
The following procedure is taken in order to eject an ink droplet from the nozzle 3 toward a sheet of paper. Firstly, the drive voltage Vp applied to the piezoelectric element 9 at time 0 is reduced to 0 (Vp=0). Accordingly, the piezoelectric element 9 is released from the state in which the piezoelectric element 9 is contracted in the planar direction and the vibrating plate 7 is released from the bent state. As a result, the capacity of the pressure chamber 2 increases by a predetermined amount and the meniscus of the ink in the nozzle 3 is pulled toward the pressure chamber 2 by a distance corresponding to the amount of increase in the capacity of the pressure chamber 2. In this process, the volume velocity of the ink in the nozzle 3 temporarily increases in the negative direction, as shown by the curve L in FIG. 12, and then gradually decreases and approaches 0 (time P1). This time period corresponds to substantially half of the natural vibration period of the ink.
Secondly, when the volume velocity of the ink in the nozzle 3 is substantially equal to 0 (time P1), the drive voltage Vp is increased to VH again (Vp=VH) so that the piezoelectric element 9 is contracted in the planar direction and the vibrating plate 7 is bent. As is clear from the curve Vp, the above-described operation corresponds to an operation in which the drive voltage Vp is applied to the piezoelectric element 9 in the form of a drive-voltage pulse wave having a pulse width of about ½ of the natural vibration period of the ink.
Accordingly, the vibrating plate 7 is bent and the capacity of the pressure chamber 2 is reduced at the time when the meniscus of the ink in the nozzle 3 is about to return to the end of the nozzle 3 after being maximally pulled toward the pressure chamber 2 and being set to a stationary state (i.e., a state in which the volume velocity is 0). Therefore, the ink in the nozzle 3 receives the pressure of ink pushed out of the pressure chamber 2 and is accelerated toward the end of the nozzle 3. As a result, the ink largely projects outward from the end of the nozzle 3 (time P2). The volume velocity of the ink in the nozzle 3 temporarily increases in the positive direction, as shown by the curve L in FIG. 12, and then gradually decreases and approaches 0 (time P3). The ink which projects outward from the end of the nozzle 3 has a substantially columnar shape. Therefore, the ink in this state is generally called an ink column. After the volume velocity of the ink in the nozzle 3 reaches 0, the direction of the pressure wave of the ink changes to the direction toward the pressure chamber 2. Therefore, the ink column, which maximally projects outward from the end of the nozzle 3, is separated from the ink in the nozzle 3, and is ejected as an ink droplet. The ejected ink droplet is caused to land on a sheet of paper.
As shown in FIG. 1, for example, in an actual inkjet head, a plurality of printing units, each of which includes the pressure chamber 2, the nozzle 3, the piezoelectric element 9 and the vibrating plate 7, etc. as above-mentioned, are generally formed on a single substrate 1. The printing units are selectively operated at a predetermined driving frequency in accordance with data corresponding to an image to be formed, so that ink droplets are selectively ejected from the nozzles 3 in the printing units to form dots on a sheet of paper. This operation is repeated to form the image on the sheet of paper. Therefore, operation intervals of the printing units are not uniform. For example, some printing units may be operated every driving cycle, and other printing units may be operated after being at rest for a relatively long time after being operated once.
In the pull-push driving method, the above-described standby state (i.e., the state in which a constant drive voltage is applied to the piezoelectric element 9 so that the capacity of the pressure chamber 2 is reduced) is continuously set for each of the printing units other than the printing units to be operated. Accordingly, in each of the printing units other than the printing units to be operated, the ink is prevented from being ejected from the end of the nozzle 3 as an ink droplet. In the standby state, the ink and the meniscus are stationary. If the standby state is set for a long time, components such as the solvent included in the ink evaporate and the viscosity of the ink increases in an area near the ink meniscus, which is the boundary between the ink and the surrounding air. As a result, it becomes difficult to reliably eject the ink droplets. In addition, there is a risk that the nozzles 3 will be clogged and, therefore, that the ink cannot be ejected from the nozzles 3. This problem is particularly severe in the case where ink containing a highly volatile solvent is used to improve the drying performance of the dots formed on the sheet.
To prevent the viscosity of the ink from being increased or to cancel the increase in viscosity if the viscosity is increased, a technique has been proposed in which a small fluctuating voltage is applied to the piezoelectric element 9 in a standby state. As a result, the vibrating plate 7 slightly vibrates without causing the ink to be ejected, thereby stirring the ink in the pressure chamber 2.
In addition, the inventor of the present invention has proposed a preferable technique in which a basic pulse with substantially the same period as the natural vibration period of the ink is generated and a fluctuating voltage based on the basic pulse is applied to the piezoelectric element 9.
The inventor of the present invention has disclosed a basic pulse preferable for vibrating the meniscus without causing the ink droplet to be ejected from the nozzle 3.